There has been some controversy regarding the classification of Stenopelix. It was initially suggested to be a pachycephalosaur, due to the apparent exclusion of the pubis from the acetabulum, and the presence of strong caudal ribs. Later, however, it was shown that the "pubis" was actually part of the acetabulum, and the so-called "caudal ribs" were sacral ribs. The curvature of the ischium and absence of an obturator foramen were not characteristics seen in other pachycephalosaurs.

In 1855, in a sandstone quarry near Bückeburg on the Harrl heights, a fossil was found of a small dinosaur. Most of its bones were in a poor condition and removed on preparation, leaving two sets of hollow impressions on the plate and counterplate. The two plates do not overlap completely. The hollows, serving as a natural mold, have since been used to produce several casts in gypsum and latex to facilitate the study of the specimen. It was originally part of the collection of Max Ballerstedt preserved in the Bückeburg Gymnsasium Adolfinum[2] but was in 1976 moved to the Georg-August-Universität Göttingen were it now resides in the collection of the Geowissenschaftliches Zentrum der Universität Göttingen.

In 1857, based on this fossil, Christian Erich Hermann von Meyer named the type speciesStenopelix valdensis.[3] The generic name is derived from Greek stenos, "narrow", and pelyx, "pelvis". The specific name refers to the Wealden Formation. The holotype, GZG 741/2 (earlier GPI 741-1, 2), found in the Obernkirchen Sandstein Formation, consists of the impressions of an almost complete skeleton, lacking the skull and the neck.